This invention relates to a process for the conversion of carbohydrates from any of a number of sources into ethanol for fuel or chemical use. The invention uses a combination of fermentation and chemical conversion to greatly increase the yield of ethanol from carbohydrates compared to the current art. In addition a high value coproduct may be produced for use in animal feed.
Ethanol is a major chemical used in human beverages and food, as an industrial chemical, and as a fuel or a component in fuels, such as reformulated gasoline to reduce emissions from automobiles. This invention relates mainly to the production of ethanol for use as a chemical or fuel.
There are several traditional ethanol processes based on fermentation of carbohydrates. In the most typical process, a carbohydrate derived from grain is hydrolyzed to its component sugars and fermented by yeast to produce ethanol. Carbon dioxide is generated in the process from a fraction of the carbohydrate by the metabolism of the yeast. The generation of carbon dioxide is inherent in the metabolism of the yeast. This production of CO2 by yeast limits the yield of ethanol from yeast to about 52% maximum on a weight basis. This is a major limitation on the economic production of ethanol as the CO2 is of low value and is typically wasted into the atmosphere and may become a burden on the environment.
In addition, yeast have a limited ability to utilize sugars other than glucose. While glucose is the major sugar produced from the hydrolysis of the starch from grains, it is not the only sugar produced in carbohydrates generally. A large research effort has gone into the potential conversion of biomass into ethanol. Biomass in the form of wastes from agriculture such as corn stover, rice straw, manure, etc., and biomass crops such as switch grass or poplar trees, and even municipal wastes such as newspaper can all be converted into ethanol. However a major limitation of these processes is the complexity of the hydrolyzate that results from treatment of the biomass to produce the fermentation medium. The hydrolyzate typically contains glucose, but also large amounts of other sugars such as xylose, which yeast cannot metabolize. This is another potential yield limitation on yeast based ethanol processes.
Research has been directed ti the use of organisms other than yeast which in contrast to yeast, do consume many if not most of the sugars derived from the hydrolysis of biomass. Examples include Zymomonas sp. bacteria and E. coli bacteria which have been genetically engineered to utilize xylose. Thereby the potential range of substrate sugars which can be converted to ethanol has been increased. There is a class of organism that has been proposed for the production of ethanol, typically of the Clostridium Sp. These thermophiles usually produce both acetic acid and ethanol. However, it is believed that these organisms produce a limited yield of ethanol. It is generally assumed in the literature on ethanol fermentation that this yield limitation is fixed by the biochemical pathway called the Embden-Myerhof pathway by which ethanol is produced in all of the organisms so far proposed for production of ethanol, including the thermophiles.
Thus none of this development has addressed the inherent problem of the yield of ethanol from sugar based on the coproduction by the organisms of CO2.
An important part of the commercial processes for producing ethanol is the production of valuable coproducts mainly for use in animal feed or food. In the corn dry milling process the coproducts include distillers dried grains and solubles (DDG, DDGS). In the corn wet milling process the coproducts include germ, gluten meal and fiber. These coproducts find large markets in the animal feed business. However in both processes to a very large extent, the ingredients in the original grain, that is the oil, protein and fiber fractions, are passed through the processes unchanged in composition, while the carbohydrate fraction is converted largely to ethanol. Therefore the value of these coproducts is based on the inherent composition of the plant components.
There are other chemicals that can be produced by industrial fermentation from carbohydrates besides ethanol. Major examples are acetic acid and lactic acid. Acetic acid is a major food ingredient in the form of vinegar and a major industrial chemical. Vinegar for food use is typically produced from potable ethanol by the action of Acetobacter sp. which oxidize ethanol to acetic acid using oxygen from the air.
Major industrial uses for acetic acid are as a solvent, as an intermediate in the synthesis of other chemicals such as vinyl acetate and in the production of cellulose acetate. Major new uses for acetic acid have been proposed such as the production of calcium magnesium acetate (CMA) for use as a road deicer in place of sodium chloride (NaCl). CMA has a much reduced environmental impact compared to NaCl since it is much less corrosive and is biodegradable.
Researchers have proposed the production of industrial grade acetic acid by fermentation from carbohydrates. However no production by fermentation currently exists due to economic factors related mainly to recovering acetic acid from dilute fermentation broths. Acetic acid is typically produced at low concentrations of around 5% or less in water as a fermentation broth. Since acetic acid has a higher boiling point than water, all of the water, about 95% of the broth, must be distilled away from the acetic acid to recover the acid or other more complex processes must be used to recover the acetic acid.
Related to this field of acetic acid production is the use of so called acetogens, a class of bacteria which utilize a unique biochemical pathway to produce acetic acid from sugars with 100% carbon yield. For example, one mole of glucose can be converted to three moles of acetic acid by Clostridium thermoaceticum. These bacteria internally convert CO2 into acetate. These bacteria are called homofermentative microorganisms or homoacetogens. They do not convert any of the carbohydrate to CO2 and only produce acetic acid. Examples of homoactogens are disclosed in Drake, H. L. (editor), Acetogenesis, Chapman and Hall, 1994, which is incorporated herein by reference in its entirety. In addition these homofermentative organisms typically convert a wide range of sugars into acetic acid, including glucose, xylose, fructose, lactose, and others. Thus they are particularly suited to the fermentation of complex hydrolyzates from biomass. However this line of research has not overcome the economic limitations of the acetic acid fermentation process to make it competitive with the natural gas based route.
Therefore, industrial acetic acid is today made from coal, petroleum or natural gas. The major process is the conversion of natural gas to methanol and the subsequent carbonylation of the methanol using carbon monoxide directly to acetic acid. U.S. Pat. No. 3,769,329 describes this process.
Related to the natural gas route, it has been proposed to produce ethanol from acetic acid by way of synthesis of esters of acetic acid produced in this process, or a related modification, and subsequent hydrogenation of the esters. U.S. Pat. Nos. 4,454,358 and 4,497,967 disclose processes to produce acetic acid from synthesis gas, which is then esterified and hydrogented to produce ethanol, and are incorporated herein by reference in their entirety. The hydrogenation of esters to produce alcohols is well known. None of these processes are based on the conversion of carbohydrates to ethanol.
There is another class of well known fermentations that have the property of converting carbohydrates at 100% carbon yield, using homofermentative lactic bacteria. These bacteria convert one mole of glucose for example into two moles of lactic acid. The relevance of this is that lactic acid may also be used as the substrate for fermentation to acetic acid by homofermentative acetogens again with 100% carbon yield. Two moles of lactic acid are converted into three moles of acetic acid by Clostridium formicoaceticum for example. Prior to the present invention, no one has been known to have devise a process to produce ethanol in high yield from carbohydrates, which is the main objective of this invention.
In accordance with one embodiment the present invention, carbohydrates are converted to ethanol with very high carbon yield by a combination of fermentation and chemical conversion, thus overcoming the major limitation of known processes for the conversion of carbohydrates to ethanol. The present invention combines several chemical and biochemical steps into a new process with many advantages. The basic process of this invention comprises three steps:
1. Converting a wide range of carbohydrates, with very high carbon yield ( greater than 90% potentially) using a homoacetic fermentation (or a combination of homolactic and subsequent homoacetic fermentations) into acetic acid,
2. Recovering, acidifying (if necessary), and converting the acetic acid to an ester (preferably, the ethyl ester using recycled ethanol product), and
3. Hydrogenating the ester, producing ethanol, and regenerating the alcohol moiety of the ester.
The net effect of this process is to convert carbohydrates in very high carbon yield to ethanol. No CO2 is produced from carbohydrates as a byproduct of this process.
Another benefit of the current invention is the production of a higher value byproduct due to the conversion of the plant proteins into bacterial single cell protein. The conversion of the plant protein into single cell bacterial protein increases the concentration of the protein, restructures the protein to have a more valuable composition for animal feed in terms of essential amino acids, for example, and potentially provides other benefits, for example, in milk production.
The conversion of the fiber fraction, and the cellulose and xylan fractions of the grain contributes to the overall yield of ethanol.
While the production of single cell protein and the utilization offiber are important additional benefits of the invention, the yield factor alone is a major improvement and can be practiced on its own in conjunction with the corn wet milling process, without the production of single cell protein or the utilization of cellulose fiber.
Advantages of the invention over the current state of the art can include one or more of the following:
1. Very high yield of product from raw material with obvious economic benefits compared to known ethanol processes,
2. No production of CO2 from carbohydrate by the process with benefits to the environment, i.e. the much more efficient conversion of renewable resources to ethanol,
3. Inherently wide substrate range for ethanol production, i.e. a wide range of potential biomass sources and their component sugars, and
4. High value byproducts, e.g., single cell protein; restructuring of plant protein, produced with high efficiency.
In one embodiment of the present invention a method to produce ethanol with a very high yield is provided. The method includes the steps of fermenting a medium which contains a carbohydrate source into acetate, acetic acid or mixtures thereof The acetate, acetic acid, or mixtures thereof are chemically converted to ethanol. Preferably, at least about 60%, more preferably at least about 80% and more preferably at least about 90% of the carbon in the carbohydrate source is converted to ethanol. Essentially none of the carbon in the carbohydrate source is converted into carbon dioxide. However, if hydrogen is produced later in the process by steam reforming, carbon dioxide will be produced at that stage. Preferably, the fermentation medium comprises less than about 20% nitrogen and yields a biomass byproduct which is useful as an animal feed, with preferably at least about 10% by weight biomass product. The carbohydrate source can include any appropriate source such as corn, wheat, biomass, wood, waste paper, manure, cheese whey, molasses, sugar beets or sugar cane. If an agricultural product such as corn is employed, the corn can be ground to produce corn and corn oil for recovery. The carbohydrate source, e.g., corn, can be enzymatically hydrolyzed prior to fermentation. Preferably, the fermentation is conducted using a homofermentative microorganism. The fermentation can be a homoacetic fermentation using an acetogen such as a microorganism of the genus Clostridium, e.g., microorganisms of the species Clostridium thermoacelicum or Clostridium formicoaceticum. 
In an embodiment of the present invention, the fermentation includes converting the carbohydrate source into lactic acid, lactate or mixtures thereof by fermentation and subsequently converting the lactic acid, lactate or mixtures thereof into acetic acid, acetate or mixtures thereof by fermentation. The lactic acid fermentation can be a homolactic fermentation accomplished using a microorganism of the genus Lactobacilus. Alternatively, the carbohydrate source can be converted into lactic acid, lactate, acetic acid, acetate or mixtures thereof in an initial fermentation using a bifido bacterium. Typically, one mole of glucose from the carbohydrate source is initially converted to about two moles lactate and the lactate is converted to about three moles acetate.
Acetic acid which is formed in connection with the fermentation can be in the form of acetate depending on the pH of the fermentation medium. The acetate can be acidified to form acetic acid. For example, the acetate can be reacted with carbonic acid in and an amine to form calcium carbonate and an amine complex of the acetate. The amine complex can be recovered and thermally decomposed to regenerate the amine and form acetic acid. The calcium carbonate can be recovered for reuse. The acetic acid can be esterified and hydrogenated to form an alcohol. Alternatively, the acetic acid may be directly hydrogenated to form ethanol. The esterification is preferably accomplished by reactive distillation.
In another embodiment of the present invention, the acetate can be acidified with carbon dioxide to produce acetic acid and calcium carbonate and esterified to acetate ester for recovery. Preferably, the process takes place at low or nearly atmospheric pressure. Preferably, the calcium carbonate is recycled to a fermentation broth in order to maintain a desired pH. Preferably, the ester is a volatile ester. As used herein, the term xe2x80x9cvolatile esterxe2x80x9d means that the ester is capable of recovery by distillation, and therefore the ester should be more volatile than the water from which it is recovered. The alcohol employed in the esterification is preferably methanol, ethanol or mixtures thereof The ester is preferably recovered by distillation, such as by reactive distillation, and subsequently converted to ethanol.
The reactive distillation can be accomplished by acidifying, esterifying and recovering the ester in a reaction column. A dilute solution of acetate salt in water mixed with ethanol is introduced near the top of a reaction section of the column. Carbon dioxide gas is introduced near the bottom of the reaction section of the column. The carbon dioxide reacts with acetate salt and ethanol in the reaction zone to form calcium carbonate and ethyl acetate. Ethyl acetate can be concentrated, e.g., by vaporizing a mixture containing excess ethanol in water and an azeotrope comprising ethyl acetate, water and ethanol. The azeotrope can be separated from the excess ethanol and water, e.g., by the addition of water, thereby causing a phase separation between an ethyl acetate-rich portion and a water and ethanol-rich portion. The ethanol and water can be returned to the reaction zone and the calcium carbonate can be recycled to a fermentation broth to control pH.
In one embodiment of the present invention, ethanol is produced from a carbohydrate source, with essentially none of the carbon and the carbohydrate source converting to carbon dioxide.
In another embodiment of the present invention, ethanol is produced from a carbohydrate source wherein at least 60%, preferably 70%, more preferably 80% and more preferably 90% and more preferably 95% of the carbon in the carbohydrate source is converted to ethanol.
In accordance with another embodiment of the present invention, an ester is recovered from a dilute solution of a carboxyic acid salt. The carboxylic acid salt is acidified with carbon dioxide to produce the corresponding carboxylic acid and calcium carbonate, and simultaneously esterified with an alcohol to form an ester. The ester is recovered. Preferably, the ester is a volatile ester and the alcohol is methanol, ethanol or mixtures thereof The ester can be recovered by distillation, such as by reactive distillation. The ester can be converted to ethanol. The acidification, esterification and recovery can take place in a reaction column. Initially, a dilute solution of the carboxylic acid salt in water mixed with alcohol is introduced near the top of a reaction section of the column. Carbon dioxide gas is introduced near the bottom of the reaction section of the column. The carbon dioxide and carboxylic acid salt and alcohol react to form calcium carbonate and a volatile ester of the carboxylic acid salt. The ester can be concentrated by vaporizing a mixture containing excess alcohol and water and an azeotrope made up of the ester, water and alcohol. The azeotrope can be separated from the excess alcohol and water, e.g., by the addition of water, thereby causing a phase separation between an ester-rich portion and a water and alcohol-rich portion. The excess alcohol and water can be returned to the reaction zone.
In accordance with an embodiment of the present invention, a carbohydrate source and natural gas are converted to an easily transportable liquid product. The carbohydrate source is converted to acetic acid, acetate or mixtures thereof by fermentation. The acetic acid, acetate or mixtures thereof is converted to ethyl acetate. At least part of the ethyl acetate is converted to ethanol using hydrogen obtained from the natural gas source. The ethanol and/or ethyl acetate which is produced is then transported to a location remote from where it is produced. Preferably, the carbohydrate source and natural gas source are located within a distance that makes it economically feasible to produce the transportable liquid product, and the remote location is a sufficient distance away that it is not economically feasible to transport the carbohydrate and natural gas to the remote location for processing. Preferably, the economically feasible distance is less than about five hundred miles and the uneconomical remote distance is greater than about a thousand miles. For example, the natural gas source and carbohydrate source can be located on a Caribbean island such as Trinidad and the remote location can be on the Gulf Coast, such as the Texas Gulf Coast. Alternatively, the carbohydrate source and the natural gas source can be located in Australia and/or New Zealand and the remote location can be Asia, e.g., Japan, Taiwan, Korea or China.
In another embodiment of the present invention at least 80% of the carbon in a carbohydrate source is converted into ethanol. The method includes enzymatically hydrolyzing the carbohydrate source to sugars and amino acids. A carbohydrate, sugars and amino acids (from the original source or another source) are converted into lactic acid, lactate or mixtures thereof by homolactic fermentation. The lactic acid, lactate or mixtures thereof are converted into acetic acid, acetate or mixtures thereofby homoacetic fermentation. The pH of the fermentation broths are maintained in a range from about pH 6 to about pH 8, using a base. A biomass byproduct which is useful as an animal feed can be recovered from the fermentation. The acetate is acidified with carbon dioxide to produce acetic acid and calcium carbonate and the acetic acid is simultaneously esterified with an alcohol to form a volatile ester. The volatile ester can be recovered using reactive distillation. Hydrogen can be produced by any number of methods, e.g., steam reforming of natural gas. The acetate ester is hydrogenated to form ethanol.